It is often desirable to measure properties of downhole fluids in situ, as this permits a driller or field engineer to measure and monitor the state (e.g., liquid or gas) and type (e.g. water or hydrocarbon) of such fluids as they exist in the formation or at a specific point in the well. Once such fluids have mingled with other fluids and/or flowed to a different temperature and pressure environment, they may undergo state and chemical changes that make it difficult for the driller or engineer to discern the original state and location of the downhole fluids. Moreover, these changes may be undesirable (e.g., scaling, deposition) and preventable. Accordingly, there exist a number of sensors for measuring in situ fluid properties.
Electrical resistivity is one fluid property that can be helpful. The resistivity of connate water tends to be very low due to the presence of solute ions, whereas the resistivity of hydrocarbons or gases is generally much higher. Thus resistivity of a formation fluid can be indicative of commercial value (e.g., hydrocarbon reservoirs). Similarly, the resistivity of a borehole fluid can be indicative of elevated danger, as a sudden change in drilling fluid conductivity would likely represent an influx of fluid from a high pressure formation.
A borehole, particularly during drilling operations, poses many challenges as an operating environment for a sensor. The sensor may be exposed to elevated temperatures and pressures, severe vibration, and in many cases prolonged exposure. In the case of a wireline tool which is re-used in different wells, the temperature and pressure conditions are cycled. In such circumstances, the components and seals of a sensor can degrade rapidly. Existing resistivity sensors leave much room for improvement.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereof do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.